A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonance frequencies, with greater amplitude than at others. Resonators can be, for example, crystal oscillators (also known as quartz oscillators), inductance-capacitive (LC) oscillators, resistance-capacitive (RC) oscillators, and Microelectromechanical systems (MEMS) oscillators, also referred to as micromechanical MEMS oscillators. Resonators are generally passive devices that are combined with active circuitry to create an oscillator. The oscillator produces a signal at the resonant frequency. A crystal oscillator, for example, is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. Crystal oscillators, such as quartz oscillators, are commonly used to generate frequencies to keep track of time (as in quartz clocks) or to generate a clock signal for digital integrated circuits. Usually, a different crystal is required for each desired frequency. Also, the crystal and the oscillator circuit compliments are typically distinct from one another, i.e., they are not integrated.
For the past several years, MEMS structures have been playing an increasingly important role in consumer products. For example, MEMS devices may be used in place of crystal oscillators to keep track of time and to generate a stable clock signal for digital integrated circuits. As these technologies mature, the demands on precision and functionality of the MEMS structures have escalated. For example, optimal performance may depend on the ability to fine-tune the characteristics of various components of these MEMS structures. Furthermore, consistency requirements for the performance of MEMS devices (both intra-device and device-to-device) often dictate that the processes used to fabricate such MEMS devices need to be extremely sophisticated.
In certain applications, the temperature stability and initial accuracy of resonators is particularly important, especially MEMS resonators. Uncompensated MEMS resonators have a temperature coefficient that can be approximately forty parts per million per degrees Celsius (i.e., 40 ppm/° C.), whereas quartz oscillators can be approximately 0.035 ppm/° C. without any special compensation. For example, in the context of sleep clock applications, which use resonators with inherent accuracy of +/−20 ppm, quartz oscillators have tighter initial accuracy, smaller temperature drift, and can be fine tuned with capacitive pulling in the oscillator circuit, as compared to uncompensated MEMS oscillators. Some conventional approaches have been used in quartz oscillators to improve initial accuracy and temperature stability. One such conventional approach uses a varactor to pull a sleep clock resonant frequency to improve temperature stability, such as described in U.S. Pat. No. 6,160,458. Another conventional approach uses pulse skipping and pulse addition to adjust the clock frequency in a receiver in order to synchronize the clock with a received signal, such as described in U.S. Pat. No. 6,167,097. Another conventional approach uses pulse skipping to create multiple clock frequencies, as described in U.S. Pat. No. 4,344,036. None of these approaches is used in the context of MEMS oscillators. Also, since uncompensated quartz oscillators have a lower temperature coefficient than MEMS oscillators, these conventional approaches are not used in a wide range of temperatures for temperature compensations of the oscillator.
Furthermore, traditional electrostatic pulling is not effective in high-frequency MEMS oscillators. High-frequency MEMS resonators, such as MEMS resonators having approximately 1 MHz frequency or greater, for example, have a very high equivalent stiffness that causes them to have a very small electrostatic frequency pulling range. In MEMS oscillators, capacitive pulling, like used in quartz-based oscillators, may also not be effective to adjust the output frequency for both initial accuracy and temperature stability due to extremely small effective capacitance of the MEMS resonators. For these reasons, new methods must be used to adjust the output frequency for both initial accuracy and temperature compensation over a wide range of temperatures for all types of resonators, such as quartz-based and MEMS oscillators.